Hmmm...haven't had a problem in my B-I-L's Concorde
His amp allows you to select low pass or high pass filtering.
Crutchfield
"The value of speaker-level inputs
Factory radios do not have dedicated outputs for amplifiers (preamp outputs). So you'll want an amplifier with speaker-level (often called high-level) inputs; these inputs enable you to tap into the factory speaker wires for a signal flow.
They're called high-level inputs because the voltage level is significantly higher than with a standard preamp output connection. These inputs convert the high voltage to one the amplifier can handle. Once connected, you'll hear clean, well-defined sound (including the lyrics to your favorite song)!
Speaker-level inputs are a standard of many two- and four-channel, and
mono subwoofer amplifiers."
